It is been found that light energy of wavelengths between approximately 400 and 530 nanometers undergoes least attenuation in deep ocean waters. In the shallower coastal ocean waters of moderate depths light energy of the wavelengths of approximately 470 to 530 nanometers is least attenuated. Accordingly, for best transmittance of underwater light energy signals, a selected source having its principal peak energies near or at the 530 nanometer wavelength region is a preferable compromise between the attenuations imposed by both coastal ocean waters and deep
In such light energy signal systems emplpoying a source which will produce its peak energy outputs in the 530 nanometers spectral region, it is most desirable to employ filters which absorb substantially all of the background energy outside the 530 nanometer spectral bandwidth region.
Narrowband optical filters which are customarily in the present state of the art have some serious disadvantages however. For example, a multi-layer optical interference filter provides a relatively narrow bandwidth of transmission but, the transmissivity of light energy through such a interference filter is drastically altered with the change of angle of incidence of the light energy entering the filter. This shift may be calculated by the following formula: EQU .lambda..sub..phi. = .lambda..sub.o (.sqroot.1 - (sin.sup.2 .phi./n.sup.2)
.lambda..sub..phi. = peak wavelength at angle PA1 .lambda..sub.0 = peak wavelength at normal incidence PA1 .phi. = angle of incidence PA1 n = effective index of refraction of the filter
For example, for n = 1.6 at 35.degree. incidence, the shift to shorter wavelengths is around 36 nanometers with significant decrease in the optical transmission of the filter. Consequently, assuming N = 1.6 at a 35.degree. angle of incidence, the shift of the bandwidth permitting transmittance will be to a shorter wavelength region by about 36 nanometers with an accompanying significant decrease in the overall optical transmission of the filter.
Another form of narrowband optical filters are the Fabry-Perot etalons. Although this type of filters can provide narrow transmittance bandwidths, they can only be most effectively employed where strong, extremely well-collimated light signals are received. Moreover, this type of filter is also very strongly temperature dependent in that their transmittance bandwidth can be shifted and altered radically as a result of significant shifts in temperature.
Grating spectrometers have also been employed as narrowband optical filters. However, this type of filter has relatively high resolution in the visible wavelength region but very small acceptance angles and extremely narrow entrance slits. Moreover, grating spectrometer type of filters are relatively large and expensive which is an undesirable disadvantage.
The Christiansen filter also provides narrowband optical filtering. This type of filter is made up of a solid pack of optical glass particles (approximately 0.5 to 2.0mm in size) in a glass cell with the glass particles immersed in a liquid of similar index of refraction but with a widely different dispersion isolating the narrow spectral regions. In the spectral region where the indices of the solid particles and the liquid are the same, a beam of light will be transmitted with little loss; where the indices differ, however, the light will be scattered out of the beam. A significant disadvantage of this type of filter is its extreme sensitivity to slight temperature changes. This problem results from the fact that the refractive index of the liquid changes more rapidly with its temperature in comparison with the refractive index change of the glass particles. Accordingly, the percentage transmission at the peak wavelength decreases as the number of interfaces in the filter is increased and therefore the wavelengths of light energy transmitted by the filter will vary considerably.
The birefringent Lyot type of filter also provides narrowband optical transmittance. This type of filter makes use of the rotation of polarization for wavelength selection and has a very narrowband resolution of 0.5A. Undesirably, however, this type of filter has a relatively restrictive acceptance angle of only several degrees and requires a temperature control to within 0.5.degree. C.
Optical filtering is also provided by a selective specular reflection type of filter employing metallic vapors. However, this type of filter is required to be maintained at several hundred degrees centigrade to sustain vapor temperature and provides only an acceptance angle of a few degrees.
The resonance fluorescence filter is based on the employment of selective absorption and reradiation of the received signal in the form of fluorescence. This technique requires an oven or cathodic sputtering in an electric discharge to maintain the required vapor pressure of a selected material such as sodium, for example, which may be used as the resonance detector. Undesirably, the associated required power supply adds to its complexity as well as expense and size, all of which contribute significant disadvantages.
The acousto-optic phenomena can also be employed to provide narrowband optical filtering. This type of filter customarily comprises a crystalline solid material which functions as a tunable device responsive to the change of frequency of an applied electrical signal to vary the frequency of a resultant electric field. One type of acousto-optical tunable filter employs a collinear configuration. More recently, a newer type of tunable acousto-optic filter has been devised employing a non-collinear acousto-optical configuration. Although this latter type of acousto-optic filter configuration holds much potential for future use, its costs as well as advantages and disadvantages are not clearly defined, nor well known for practical applications at this time.
It is highly desirable for use in underwater ocean optical signal systems that a wide-angle, narrowband optical filter be devised which is small in size, compact and rugged in construction, and economical to fabricate, as well as convenient to use.